Document `RUSTDOCFLAGS`

diff --git a/src/doc/environment-variables.md b/src/doc/environment-variables.md
index 21274f9f31a822c76e8f49dacd59de57fda443fc..6fe532d61a4487eb8fc5cb8a37f719bd534e49f6 100644 (file)
--- a/src/doc/environment-variables.md
+++ b/src/doc/environment-variables.md
@@ -22,6 +22,9 @@ system:
   invocation, with the first argument being rustc.
 * `RUSTDOC` - Instead of running `rustdoc`, Cargo will execute this specified
   `rustdoc` instance instead.
+* `RUSTDOCFLAGS` - A space-separated list of custom flags to pass to all `rustdoc`
+  invocations that Cargo performs. In contrast with `cargo rustdoc`, this is
+  useful for passing a flag to *all* `rustdoc` instances.
 * `RUSTFLAGS` - A space-separated list of custom flags to pass to all compiler
   invocations that Cargo performs. In contrast with `cargo rustc`, this is
   useful for passing a flag to *all* compiler instances.